This invention relates to electronic measuring instruments, and more particularly to digital display circuitry for such instruments. In the prior art, a wide variety of electronic measuring instruments having digital readouts exist. These include for example, dual slope volt meters, ohmmeters, current meters, and frequency meters.
Many differences in design exist among these various meters. Basically, however, they each include a counter, which generates counts during spaced apart time intervals, that are representative of the measurements made. Each of these counts are stored in a register; and the output of the register drives a digital display.
One problem however, with such a digital display, is that the displayed characters tend to be unstable. That is, due to a variety of conditions, the displayed characters tend to flicker about some nominal or average value. This flickering is distracting and also causes the display to be difficult to read.
One source of this display instability is random noise on the input signal. Random noise causes intermittent variations in the input signal, which in turn causes intermittent variations in the counts that are generated. Thus the display flickers because all of the different counts are transferred into the register.
Variations in the time base generator can also cause flickering in the display. That is, small variations in the time intervals during which the counter is enabled produces corresponding variations in the counts generated.
These and other sources of display instability are particularly distracting when the signal being measured has a nominal value that makes several of the least significant digits of the display equal to 9. In such a case, intermittent noise and time base errors that cause a variation of only one bit in the counter, produces a flickering in all of the 9 digits in the display.
In the prior art, one approach that has been taken to solve the above problem is to perform a time averaging on the contents of the operational counter, and to display only the time averaged count. For example, four consecutive measurements may be made; and the average of these measurements can be calculated by dropping the least significant two bits. Such time averaging does in fact reduce flicker in the display.
A problem however, with such time averaging, is that the instrument cannot readily track a rapidly changing input signal. For example, if four consecutive measurements are time averaged, then the instrument will update its display at only 1/4 the rate at which it would if no averaging were used. Thus, in the prior art, a trade off existed between display stability and ability to track rapidly changing signals.
Accordingly, it is one object of the invention to provide an improved display circuit for an electronic measuring instrument.
Another object of the invention is to provide a stabilized display for an electronic measuring instrument which does not time average.
Still another object of the invention is to provide a stabilized display for an electronic measuring instrument which can readily track rapidly changing input signals.